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Homeostatic mechanisms in mammals respond to hormones and nutrients to maintain blood glucose levels within a narrow range. Caloric restriction causes many changes in glucose metabolism and extends lifespan; however, how this metabolism is connected to the ageing process is largely unknown. We show here that the Sir2 homologue, SIRT1—which modulates ageing in several species^,, —controls the gluconeogenic/glycolytic pathways in liver in response to fasting signals through the transcriptional coactivator PGC-1α. A nutrient signalling response that is mediated by pyruvate induces SIRT1 protein in liver during fasting. We find that once SIRT1 is induced, it interacts with and deacetylates PGC-1α at specific lysine residues in an NAD⁺-dependent manner. SIRT1 induces gluconeogenic genes and hepatic glucose output through PGC-1α, but does not regulate the effects of PGC-1α on mitochondrial …